| 摘要 |
PROBLEM TO BE SOLVED: To reduce weight and a cost by holding sufficient strength even if the thickness of a center drum is thinned by relieving high stress caused in a through hole formed in the center drum. SOLUTION: A connecting part 15 in a center frame 12 is formed in a box shape composed of upper and lower end plates 12a, 12a and side plates 12b, 12b extended and arranged between these both end plates 12a, 12a, and the through hole 20 for passing hydraulic piping 10 is formed in a position of an inside area of the connecting part 15 in the center drum 14. A prescribed angleθquantity is formed as a large circular arc part 20a having a large radius of curvature with a position crossing a diagonal line D being at least a high stress part as the center among this through hole 20, and a quantity other than that, that is, a residual angle quantity with the horizontal and vertical center lines H, V as the center is formed as a small circular arc part 20b having a small radius of curvature.
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